1. Field of the Invention
The present invention relates to a semiconductor printing and exposing system for maintaining a stable exposure performance in a projection optical system and a method for controlling the same.
2. Description of the Prior Art
Recently, the patterns of semiconductors such as so-called IC, LSI, VLSI and others are increasingly miniaturized and integrated. The width of line in these patterns is now being decreased to the range of one to two .mu.m. Such miniaturization and integration require an exposure apparatus which has an exposure performance capable of printing finer patterns with the width of line in the range of one to two .mu.m and an alignment performance capable of accurately aligning the patterns with one another through a plurality of steps and which can provide wafers having no defects. In order to satisfy these requirements, various types of projection and exposure systems are energetically being developed.
In such projection and exposure systems, a projection optical system has its depth of focus usually in the order of .+-.1-2 .mu.m depending on the relationship between the effective F-number and the wavelength used therein. For this reason, the projection and exposure system should have a focusing mechanism for exactly imaging the pattern of a photo-mask on the surface of a wafer. In addition, the projection optical system must inherently have its error of magnification and its distortion, but these have to be maintained less than the accuracy of alignment which is required to be in the order of .+-.0.3 .mu.m for aligning the finer patterns with one another. When the wafer is subjected to exposure operation, the projection optical system is increased in temperature by absorbing part of the heat energy from the light of exposure. As a result, the projection optical system changes in its optical performances to vary or displace its optimum image position. Thus, the magnification and distortion of the projection optical system will unavoidably be affected adversely by the variations of the optical performance.
FIG. 1 illustrates the time chart showing illumination system in the prior art system and also displacements .DELTA.x of the imaging position in the projection optical system thereof. As shown in FIG. 1, the exposure process consisting of a series of exposure steps is initiated at Time T1 and then terminated at Time T3. During a period from T3 to T4, the exposed wafer is brought out and then a new wafer is brought in. Supposing that the optimum image position of the projection optical system is the saturated position in the displacement .DELTA.x, the imaging position is gradually increased from Time T1 at which the exposure process is initiated and reaches the optimum imaging position at Time T2. During the period between T3 and T4 for which no exposure is effected, however, the displacement .DELTA.x returns to its original state. In the prior art, thus, the projection optical system thereof is changed in its performances during the entire process, leading to the fault of its focusing function and the decrease of its alignment accuracy.